A one loop type shown in FIG. 2(a) is known as a simplest system configuration of a semiconductor wafer resist stripping system which uses a persulfuric acid solution prepared by the electrolysis of a sulfuric acid solution.
In this system, an liquid outlet side of a cleaning machine 21 and an liquid inlet side of an electrolytic cell 23 are connected together by a feed line of a circulation line 31 through a cooler 24 and a storage tank 22, and an liquid outlet side of the electrolytic cell 23 and an liquid inlet side of the cleaning machine 21 are connected together by a return line of the circulation line 31 through a heater 25. Thus, the components are connected together in one loop.
This system has advantage of a reduced number of pumps and valves, but involves the following problems.
In the case where the amount of persulfuric acid required per unit time in the cleaning machine is large, that is, in the case where the ion dose for a semiconductor wafer is large or the resist thickness is large or the processing time is to be shortened, it is necessary to install a large number of electrolytic cells in order to produce a persulfuric acid solution of a high concentration. However, an increase in the amount of a sulfuric acid solution circulated leads to an increase of a cooling load on the cooler and an increase of a reheating load on the heater. If the amount of the sulfuric acid solution circulated is small, it is impossible to maintain a proper flow rate distribution of the sulfuric acid solution in the interior of the electrolytic cell.
These problems may be solved by adopting a two loops type shown in FIGS. 2(b) and 2(c) or a three loops type shown in FIG. 2(d). (See, for example, FIG. 2 in Patent Document 1.)
In the system of FIG. 2(b), a cleaning machine 21 and a storage tank 22 are connected together by a feed line of a circulation line 31a, and the storage tank 22 and the cleaning machine 21 are connected together by a return line of the circulation line 31a through a heater 25. Further, the storage tank 22 and an electrolytic cell 23 are connected together by a feed line of a circulation line 31b through a cooler 26, and the electrolytic cell 23 and the storage tank 22 are connected together by a return line of the circulation line 31b. A heater 27 is installed in the storage tank 22 to prevent a drop in temperature of a circulating solution. That is, the components are connected together in two loops, one of which is formed by the feed line of the circulation line 31a and the return line of the circulation line 31a and the other of which is formed by the feed line of the circulation line 31b and the return line of the circulation line 31b. 
In the system of FIG. 2(c), a cleaning machine 21 and a storage tank 22 are connected together by a feed line of a circulation line 31a, and the storage tank 22 and the cleaning machine 21 are connected together by a return line of the circulation line 31a through a heater 25. Further, the storage tank 22 and an electrolytic cell 23 are connected together by a circulation line 31b through a cooler 26, and the electrolytic cell 23 and the cleaning machine 21 are connected together by a circulation line 31. A heater 27 is installed in the storage tank 22 to prevent a drop in temperature of a circulating solution. That is, the components are connected together in two loops, one of which is formed by the feed line of the circulation line 31a and the return line of the circulation line 31a and the other of which is formed by the feed line of the circulation line 31a, the circulation line 31b and the circulation line 31.
In the system of FIG. 2(d), a cleaning machine 21 and a storage tank 22 are connected together by a feed line of a circulation line 31a, and the storage tank 22 and the cleaning machine 21 are connected together by a return line of the circulation line 31a through a heater 25. Further, the storage tank 22 and an electrolytic cell 23 are connected together by a feed line of a circulation line 31b through a cooler 26, and the electrolytic cell 23 and the storage tank 22 are connected together by a return line of the circulation line 31b. Further, the electrolytic cell 23 and the cleaning machine 21 are connected together by a circulation line 31. A heater 27 is installed in the storage tank 22 to prevent a drop in temperature of a circulating solution. That is, the components are connected together in three loops, which are the loop formed by the feed line of the circulation line 31a and the return line of the circulation line 31a, the loop formed by the feed line of the circulation line 31b and the return line of the circulation line 31b, and the loop formed by the feed line of the circulation line 31a, the feed line of the circulation line 31b and the circulation line 31.
Thus, in the conventional art, the storage tank was used as a reaction site for expediting a resist decomposing reaction secondarily.    Patent Document 1: Japanese Patent Laid-Open No. 2007-266495